LanthanideElement 61 · f-block

Promethium (Pm)

Promethium (Pm) is a lanthanide with atomic number 61; its most stable known isotope has mass number 145. At room temperature it is a solid that melts at 1,042 °C and boils at 3,000 °C.

61PmPromethium[145]

Atomic number

protons

Atomic weight

[145]

most stable isotope

Melting point

1,042 °C

1,315.15 K

Boiling point

3,000 °C

3,273 K

Density

7.26 g/cm³

near room temperature

Electronegativity

1.13

Pauling scale

State at 25 °C

Solid

Discovered

1945

United States (Oak Ridge)

Promethium at a glance

Identity

Symbol	Pm
Atomic number (Z)	61
Atomic weight	[145] mass number of the most stable isotope
Category	Lanthanide
Group	— (f-block: lanthanide series)
Period	6
Block	f-block
CAS number	7440-12-2

Atomic properties

Electron configuration	[Xe] 4f⁵ 6s² Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f⁵ 5s² 5p⁶ 6s²
Electrons per shell	2, 8, 18, 23, 8, 2
Oxidation states	+3 Less common: +2
Electronegativity (Pauling)	1.13
First ionisation energy	5.582 eV (538.6 kJ/mol) 2nd: 10.938 eV · 3rd: 22.44 eV
Atomic radius (empirical)	185 pm
Covalent radius	173 pm
Van der Waals radius	238 pm

Physical properties

State at 25 °C	Solid
Melting point	1,042 °C · 1,907.6 °F · 1,315.15 K
Boiling point	2,999.8 °C · 5,431.7 °F · 3,273 K
Density	7.26 g/cm³
Thermal conductivity	17.9 W/(m·K)

Isotopes & abundance

Natural occurrence	Trace in nature / mostly synthetic
Radioactive	Yes
Most stable isotope	¹⁴⁵Pm — half-life 17.7 years
Known isotopes	40

Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.

Promethium electron configuration

Promethium has 61 electrons. Written in short form, its ground-state configuration is [Xe] 4f⁵ 6s² — the electrons of the noble gas xenon, plus 4f⁵ 6s².Written out in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f⁵ 5s² 5p⁶ 6s².

4f↑↑↑↑↑

6s↑↓

Discovery and name

Discovered

1945 — Jacob A. Marinsky, Lawrence E. Glendenin and Charles D. Coryell · United States (Oak Ridge).

Name

Prometheus, who stole fire from the gods.

Known for

All its isotopes are radioactive; it hardly exists in nature. Promethium-147 has powered luminous paints and nuclear batteries.

Isotopes of promethium

Promethium has no stable isotopes and does not occur naturally in significant amounts. Its longest-lived known isotope is:

Isotope	Protons	Neutrons	Half-life
¹⁴⁵Pm	61	84	17.7 years

40 isotopes of promethium are known in our data set (half-lives as published in nuclear data tables).

Promethium and its neighbours in the lanthanide series

Neighbouring elements in the same f-block series:

Element	Z	Atomic weight	Melting point	Boiling point	Density	Electronegativity
Cerium	58	140.12	799 °C	3,443 °C	6.77 g/cm³	1.12
Praseodymium	59	140.91	931 °C	3,520 °C	6.773 g/cm³	1.13
Neodymium	60	144.24	1,016 °C	3,074 °C	7.01 g/cm³	1.14
Promethium	61	[145]	1,042 °C	3,000 °C	7.26 g/cm³	1.13
Samarium	62	150.36	1,072 °C	1,794 °C	7.52 g/cm³	1.17
Europium	63	151.96	822 °C	1,529 °C	5.24 g/cm³	1.2
Gadolinium	64	157.25	1,313 °C	3,273 °C	7.9 g/cm³	1.2

Frequently asked questions

6 answers

What is the atomic mass of promethium?

Promethium has no stable isotopes, so it has no standard atomic weight. Periodic tables show [145], the mass number of its most stable known isotope.

How many protons, neutrons and electrons does promethium have?

A neutral promethium atom has 61 protons and 61 electrons. Its most stable isotope, promethium-145, has 84 neutrons.

What is the electron configuration of promethium?

[Xe] 4f⁵ 6s² (in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f⁵ 5s² 5p⁶ 6s²).

What are the melting and boiling points of promethium?

Promethium melts at 1,042 °C (1,907.6 °F) and boils at 2,999.85 °C (5,431.7 °F).

Is promethium a metal?

Promethium is a metal. It is in the f-block, period 6, and is a solid at room temperature.

Who discovered promethium?

1945 — Jacob A. Marinsky, Lawrence E. Glendenin and Charles D. Coryell (United States (Oak Ridge)).